[PATCH weston] compositor: change rounding in weston_surface_to_buffer_rect()

[Derek Foreman]

Rounding both corners of the rectangle down can result in a 0
width/height rectangle before passing to weston_transformed_rect.

This showed up as missing damage in weston-simple-damage (the
bouncing ball would leave green trails when --use-viewport was
used)

Also, add a big fat warning for users of the function, since
some of its operation may not be obvious at a glance.

Signed-off-by: Derek Foreman <derekf at osg.samsung.com>
---
 src/compositor.c | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/src/compositor.c b/src/compositor.c
index 4895bd6..bf59fa8 100644
--- a/src/compositor.c
+++ b/src/compositor.c
@@ -932,6 +932,20 @@ weston_surface_to_buffer(struct weston_surface *surface,
 	*by = floorf(byf);
 }
 
+/* Users of weston_surface_to_buffer_rect() need to be
+ * careful - it converts to integer as an intermediate
+ * step, and rounds off at that time - the boundary may
+ * not be exactly as expected.  It works fine when used
+ * for damage tracking since a little extra coverage is
+ * not a problem.
+ *
+ * Also, since the rectangles are specified by 2 corners,
+ * if the input is not axis aligned and the surface to
+ * buffer transform includes a rotation that isn't a
+ * multiple of 90 degrees, the output rectangle won't
+ * have the same area as the input (in fact it could have
+ * none at all)
+ */
 WL_EXPORT pixman_box32_t
 weston_surface_to_buffer_rect(struct weston_surface *surface,
 			      pixman_box32_t rect)
@@ -945,8 +959,8 @@ weston_surface_to_buffer_rect(struct weston_surface *surface,
 	rect.y1 = floorf(yf);
 
 	scaler_surface_to_buffer(surface, rect.x2, rect.y2, &xf, &yf);
-	rect.x2 = floorf(xf);
-	rect.y2 = floorf(yf);
+	rect.x2 = ceilf(xf);
+	rect.y2 = ceilf(yf);
 
 	return weston_transformed_rect(surface->width_from_buffer,
 				       surface->height_from_buffer,
-- 
2.6.2